1. Field of the Invention
This invention relates to an externally mounted speed changer for a bicycle including a pantograph type mechanism by which the position of a transmission chain on a pluarlity of sprocket wheels on the hub of the rear wheel is changed from one sprocket wheel to another. More particularly, it is concerned with a device for controlling the tension of the transmission chain.
2. Description of the Prior Art
A typical speed changer for a bicycle comprises a sprocket wheel assembly 2 supported on a rear wheel hub 1, as shown in FIGS. 1 and 2. The sprocket wheel assembly 2 comprises a plurality of sprocket wheels 2a to 2f, each having a different number of teeth. A stationary member 4 has a bracket 3, and a pair of links 5 and 6 are rotatably connected at one end to the stationary member 4 by pins 7 and 8, respectively, while a movable member 9 is rotatably connected to the other ends of the links 5 and 6 by pins 10 and 11, respectively, whereby a pantograph mechanism A is defined. The pantograph mechanism A is secured to the hub 1 by the bracket 3. A guide pulley 13 is provided for guiding a transmission chain 12 about one of the sprocket wheels 2a to 2f, and a tension pulley 14 is provided for applying tension to the chain 12. The guide and tension pulleys 13 and 14 are rotatably supported by a chain positioning frame 15 which is rotatably mounted on the movable member 9. A torsion spring 16 is provided for urging the frame 15 in the direction which enables the chain 12 to be tensioned. A spring 17 is provided for urging the links 5 and 6 in the direction in which the frame 15 is urged outwardly or into vertical alignment with the sprocket wheel 2f of the smallest diameter. One end of an outer cable 18 is connected to the stationary member 4, while one end of an inner wire 19 is fastened to the movable member 9. If a speed changer operating lever provided on a handle or top tube, but not shown, is pulled, the links 5 and 6 are rotated inwardly against the force of the spring 17. This causes the inward displacement of the frame 15 toward the sprocket wheel 2a of the largest diameter, so that the chain 12 may be positioned about a selected sprocket wheel.
According to the arrangement hereinabove described, however, it is impossible to control the tension of the transmission chain 12, since the spring 16 is secured to the movable member 9 at one end, and to the frame 15 at the other end. It is difficult to choose a spring having a force which enables the driving of the chain and its positioning from one sprocket wheel to another to be achieved easily. Even if a spring having an appropriate force is employed, its fatigue results in a less strong tension which makes it easier for the chain to come off the sprocket wheel. This presents a great problem in the driving of the chain and the changing of the bicycle speed.